Most manual transmissions are composed of low-speed gears having a spline clutch connected to the gears against rotation, high-speed gears having a spline clutch connected to the gears against rotation, and a clutch hub interposed between the low-speed gears and the high-speed gears and connected an output shaft against rotation. A spline sleeve is fitted on the clutch hub for axially sliding movement. On actual speed changing, the spline sleeve fitted with the clutch hub rotatable with the output shaft is moved axially into spline engagement with either one of the low-speed gears and the high-speed gears, thereby selecting a transmission ratio in accordance with the gear ratios of the low and high gears so as to obtain the power transmission at the selected transmission ratio.
A selector mechanism of such manual transmissions as described above is of the multi-rod gear selector type in which there may be shift arms, one to each spline sleeve, and selector rods are each provided for each of the shift arms to support the shift arms and transmit the power. The shift arms are separately connected with the individual selector rods, one to each shift arm, by suitable fixing means such as bolts, spring pins, welding and the like. In the design, one shift arm and one selector rod are in combination employed for a transmission ratio between a pair of low and high gears and, therefore, the five-speed, one-reverse manual transmissions require three selector rods and also the six-speed, one-reverse manual transmissions require four selector rods. Japanese Utility Model Laid-Open No. 57819/1987 discloses an example of the multi-rod gear selector mechanism.
Another selector mechanism of the manual transmissions is of the single rod gear selector type, although being minor in use, in which all desired shift arms are arranged on a single selector rod in order to reduce the transmission in its weight, required space, the number of the parts and the like. In this design, all the shift arms except for a shift arm at a design location are arranged so as to slide on a single selector rod.
FIG. 3 is a top plan view illustrating an exemplary prior gearshift mechanism of the single rod gear selector type. The gearshift mechanism 60 in FIG. 3 has low and high gear pairs for obtaining various speed ratios or transmission ratios. Further, there are provided selector rods extending over the overall length of a transmission case 74, one to each gear pair. That is, for the six-speed, one reverse manual transmission illustrated, it is necessary to install four selector rods extending through the full length of the transmission case 74, which are a first-speed, reverse selector rod 61, a second-, third-speed selector rod 62, a fourth-, fifth-speed selector rod 63 and a sixth-speed selector rod 64. The first-speed, reverse selector rod 61 is connected to a first-speed, reverse shift arm 65. Likewise, the second-, third-speed selector rod 62, the fourth-, fifth-speed selector rod 63 and the sixth-speed selector rod 64 are connected to a second-, third-speed shift arm 66, a fourth-, fifth-speed shift arm 67 and a sixth-speed shift arm 68, respectively. Shift blocks 69 to 72, each corresponding to each of the shift arms 65 to 68, are assembled on the second-, third-speed selector rod 62. There is provided engaging means operable in response to the manipulation of a shift lever, not shown, whether the engaging means is of a direct control type in which one end of the shift lever constitutes the engaging means or a remote control type in which the shift lever is operatively connected with the engaging means through wires or rods. Manipulating the shift lever thus causes the engaging means to make selective engagement with any one of the shift blocks 69 to 72, resulting in shifting any one of the shift arms 65 to 68, selected depending on the shift direction of the engaging means, so that any desired transmission range may be selected out of six-speeds and one reverse. On the gearshift mechanism of multi-rod gear selector type designed as described above, two or more selector rods occupy the space exceeding the area for the selector rods intrinsically indispensable for the transmission. This becomes a major problem exerting the great adverse influence upon the gearshift mechanism as to production cost, weight and the desired number of parts.
In contrast, FIG. 4 is a side elevation showing an exemplary prior gearshift mechanism of single-rod gear selector type. The gearshift mechanism 80 in FIG. 4 has a single selector rod 81 supported for sliding movement by the transmission case 74 and carrying thereon all shift arms 82 to 84. Both the first-, second-speed shift arm 82 and third-, fourth-speed shift arm 83 are movable in a linearly sliding manner relatively of the selector rod 81, whereas the fifth-speed shift arm 84 is fixed to the selector rod 82. Shift block arrangement 85 is concentrated on a design location for selectively shifting the shift arms 82 to 84. In the single rod gear selector type, the permissible spaces for each of the shift arms 82 to 84 become very small and, therefore, each shift arm is inevitably made slender although such an structure is less desirable because the shift arms become reduced in stiffness. In addition, the practical gearshift mechanism 80 makes no remarkable saving in the desired space for all the parts, including the shift arms 82 to 84 and the selector rod 81, compared with the gearshift mechanism 60 of multi-rod gear selector type having four selector rods. Moreover, the need of a large number of forward speeds such as the six-speed, one reverse transmissions in recent years makes much more it difficult to assemble all shift arms on a single selector rod.
In the conventional gearshift mechanism of multi-rod gear selector type, the problem as described above arises that increased number of forward speeds increases the desired number of the selector rods. Moreover, the aggregation of the shift blocks on one location results in requiring such arrangement that the shift blocks for the shift arms corresponding to the outer opposing selector rods are to be formed on the extensions of the shift arms, which elongate beyond the inner selector rods. This causes the inevitable drawback in which the gearshift mechanism becomes greater in height so that it is hard to make it compact in size. In addition to the above, the shift blocks should be each assigned exclusively to each of the selector rods and, therefore, the parts necessary for gearshift in corresponding to the shift blocks have to be provided at every shift block. In contrast, on the prior gearshift mechanism of single rod gear selector type, the shift arms are limited in material to pressed iron alloys, cast iron alloys or the like, which is relatively higher in strength in order to avoid a deficiency in stiffness owing to the slender structure of the shift arms. Moreover, increased number of forward speed ranges causes such inconvenience that all shift arms may not be kept round a single selector rod. In recent years, accordingly, much attention has been given to a gearshift mechanism, which may contribute to make the transmissions compact in size, reduce the number of the desired parts and make available of inexpensive materials thereby being reduced in production cost.
The present invention has for its primary object to overcome the problems described above and in particular to provide a manual transmission having two selector rods, which design may reduce the desired number of selector rods by roughly half of the selector rods required in the multi-rod gear selector type while make it possible to improve the stiffness of the shift arms compared with the single rod gear selector type in which all shift arms are integrated on the single gear selector rod, whereby the layout design of the shift arms and the selector rods may afford to make the gearshift mechanism less in its height.